An approach to quantum light emitters developed at Los Alamos National Laboratory stacks two different atomically thin materials to realize a source that generates a stream of circularly polarized single photons that can be used for a variety of quantum information and communication applications. According to Los Alamos researcher Han Htoon, the work shows that it’s possible for a monolayer semiconductor to emit circularly polarized light without the need for an external magnetic field. “This effect has only been achieved before with high magnetic fields created by bulky superconducting magnets, by coupling quantum emitters to very complex nanoscale photonics structures or by injecting spin-polarized carriers into quantum emitters. Our proximity-effect approach has the advantage of low-cost fabrication and reliability.” The polarization state is a means of encoding the photon, so the achievement is an important step in the direction of quantum cryptography or quantum communication. “With a source to generate a stream of single photons and also introduce polarization, we have essentially combined two devices in one,” Htoon said. The team stacked a single-molecule-thick layer of tungsten diselenide semiconductor onto a thicker layer of nickel phosphorous trisulfide magnetic semiconductor. Using atomic force microscopy, the team created a series of nanometer-scale indentations on the thin stack of materials. The 400-nm-diameter indentations created by the atomic microscopy tool proved useful for two effects when a laser was focused on the stack of materials. First, the indentation forms a well, or depression, in the potential energy landscape. Electrons of the tungsten diselenide monolayer fall into the depression. That stimulates the emission of a stream of single photons from the well. The nanoindentation also disrupts the typical magnetic properties of the underlying nickel phosphorous trisulfide crystal, creating a local magnetic moment pointing up out of the materials. That magnetic moment circularly polarizes the photons being emitted. To provide experimental confirmation of this mechanism, the team first performed high magnetic field optical spectroscopy experiments in collaboration with National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos. The team then measured the minute magnetic field of the local magnetic moments in collaboration with the University of Basel in Switzerland. The team is currently exploring ways to modulate the degree of circular polarization of the single photons with the application of electrical or microwave stimuli. That capability would offer a way to encode quantum information into the photon stream. Further coupling of the photon stream into waveguides would provide the photonic circuits that allow the propagation of photons in one direction. Such circuits would be the fundamental building blocks of an ultrasecure quantum internet. The research was published in Nature Materials (www.doi.org/10.1038/s41563-023-01645-7).
